Chiastozygaceae


Classification: Mesozoic -> Eiffellithales -> Chiastozygaceae
Sister taxa: Chiastozygaceae, Eiffellithaceae, Rhagodiscaceae,

Short diagnosis: Murolith (loxolith type) coccoliths with variably-developed proximal/inner cycles and central-areas typically spanned by axial, non-axial or diagonal cross-bars or a single transverse bar, which is commonly formed from four fused arms/bars. LM images may be unicyclic or bicyclic.


Daughter taxa (blue => in age window 0-300Ma) Granddaughter taxa
Muroliths with complex axial cross structure or plate, typically incorporating 8 near-axial or radial bars or suture lines.

Muroliths with bicyclic rim and tranverse bar formed from laths which continue around the inner edge of the rim, delineating the two circular perforations.

Bicyclic (LM) murolith with a birefringent, spine-bearing bar. Distal/outer-cycle displays anticlockwise imbrication, unlike all other Mesozoic loxoliths. Triassic genus.

Loxoliths with central area filled with a tall, columnar to bulbous boss borne on a plate. Seen in side view. Spine has a thick wall, narrow axial canal and is formed from two crystallographically distinct units.

Loxolith coccoliths with narrow unicyclic rim and central area spanned by a plate incorporating broad, fibrous, tapering axial-cross-bars. Dark overall image in XPL.

Murolith (loxolith) coccoliths with relatively narrow rim and wide central area spanned by diagonal bars. Inner/proximal rim cycle is variably developed and so the LM image may be unicyclic or bicyclic.

Small loxolith coccoliths with multiple central area radial bars supporting a central spine.

Murolith (loxoliths) with a robust, broad, high rim and central area that may be vacant, closed or spanned by bars or grill.

Square-shaped loxolith cococoliths with cross bars that run parallel to the coccolith sides.

Bicyclic (LM) muroliths with broad, bright inner-cycle and robust, birefringent, transverse bar.

Bicyclic muroliths with axial cross and additional small bars in the central-area.

Loxolith coccoliths with broad, open central-area, probably spanned by a rarely preserved, finely perforate net

Muroliths with narrow, bicyclic rim and wide central-area spanned by axial cross bars and a perforate plate.

Muroliths with narrow, unicyclic rim and wide central-area spanned by axial, concentric and lateral bars.

Murolith (loxolith) coccoliths with broad, high rim and narrow or closed central-area, which may be spanned by bars.

Murolith with distal/outer-cycle that shows little or no imbrication and central area spanned by transverse bars; rim exhibits spiral interference pattern.

Unicyclic muroliths with very broad rim and narrow central-area spanned or filled by a tranverse bar.

Spinose forms with minute basal loxolith coccolith with axial cross bars. The spines are long, parallel sided (?or tapering) and cruciform in section.



Loxolith muroliths with relatively narrow rim and wide central area spanned by axial cross bars. The cross is usually spine bearing. Inner/proximal rim cycle is variably developed and so the LM image may be unicyclic or bicyclic.

Murolith (loxolith) coccoliths with central area spanned by complex transverse structures principally constructed from two or four projections. Delicate lateral bars may also be present but are generally only seen in exceptional preservation.

Narrowly elliptical loxolith coccoliths with a central-area structure that supports a broad, hollow, flaring spine.

Muroliths with axial, lateral and concentric bars spanning the central-area.

Murolith (loxolith) coccoliths with central area spanned by variably constructed transverse bar. The bar is usually spine bearing. Inner/proximal rim cycle is variably developed and so the LM image may be unicyclic or bicyclic. Delicate lateral bars may also be present but are generally only seen in exceptional preservation.

Taxonomy:

Citation: Chiastozygaceae Rood et al., 1973
Rank: Family
Notes & discussion: The orientation of the cross-bars has previously been used as a familial level criterion, e.g., Chiastozygaceae Rood, Hay and Barnard, 1973; Ahmuellerellaceae Reinhardt, 1965; and Zygodiscaceae Hay and Mohler, 1967; respectively.

Farinacci & Howe catalog pages: Chiastozygaceae [no catalog entry yet]

Short diagnosis: Murolith (loxolith type) coccoliths with variably-developed proximal/inner cycles and central-areas typically spanned by axial, non-axial or diagonal cross-bars or a single transverse bar, which is commonly formed from four fused arms/bars. LM images may be unicyclic or bicyclic.


Morphology remarks: The proximal, inner wall rim cycle is often low and narrow, and the rim may thus appear unicyclic in the LM. Thus, the LM discernible unicyclicity/bicyclicity of the wall is not considered as a criterion for generic subdivision of this group, although some authors have used this criterion to divide the group into separate families. The dominant, outer wall cycle is dextrally imbricate.
The family is long-ranging (Late Triassic-early Eocene) and in the Mesozoic is largely represented by relatively simply-constructed coccoliths in which iterative homology appears to have been common. Apart from a few distinctive forms, many species (often small) differ in very subtle ways and species names are poorly constrained and applied in different senses by different authors.

Geological Range:
Last occurrence (top): within Selandian Stage (59.24-61.61Ma, top in Selandian stage). Data source: Total of range of species in this database
First occurrence (base): within Norian Stage (209.46-228.35Ma, base in Norian stage). Data source: Total of range of species in this database

Plot of occurrence data:

References:

Bown, P.R., (1998). Triassic. In: Bown, P.R. (Editor), Calcareous nannofossil biostratigraphy. British Micropalaeontological Society Publication Series. Chapman & Hall, pp. 29-33.

Hay, W.W. & Mohler, H.P., (1967). Calcareous nannoplankton from Early Tertiary rocks at Point Labau, France and Paleocene-Early Eocene correlations. Journal of Paleontology, 41(6): 1505-1541.

Reinhardt, P., (1965). Neue Familien für fossile Kalkflagellaten (Coccolithophoriden, Coccolithineen). Monatsberichte der Deutschen Akademie der Wissenschaften zu Berlin, 7: 30-40.

Rood, A.P.; Hay, W.W. & Barnard, T., (1973). Electron microscope studies of Lower and Middle Jurassic coccoliths. Eclogae Geologicae Helvetiae, 66: 365-382.


nannotaxlogo400px

Chiastozygaceae compiled by Jeremy R. Young, Paul R. Bown, Jacqueline A. Lees viewed: 21-11-2017

Taxon Search:
Advanced Search

Go to Archive.is to create a permanent copy of this page - citation notes



Comments (0)

No comments yet. Be the first!

Add Comment

* Required information
1000
Captcha Image
Powered by Commentics